This invention relates generally to acoustic transducers for underwater and ultrasonic applications and more particularly to a broadband directional transducer which provides a constant beamwidth that is independent of frequency over its bandwidth and which produces both an azimuthal pressure distribution that is independent of the distance from the transducer and a beam pattern that has no side lobes.
A constant beamwidth transducer, that is, a transducer whose beam pattern is independent of frequency over a wide frequency range, is desirable for many applications in ultrasonics and underwater acoustics. Some examples of possible applications for such a transducer are: broadband echo ranging, high data rate communication, and nondestructive ultrasonic testing, medical diagnosis and materials research.
Most directional acoustic transducers and arrays exhibit beam patterns which are frequency-dependent; for example, the beamwidth of a plane piston or line array decreases with increasing frequency. As a result, the spectral content of a transmitted or received signal varies with location in the beam. Thus, the fidelity of an underwater acoustic system depends on the relative orientation of the transmitter and receiver. A broadband directional transducer having a beam pattern that is constant for all frequencies over its bandwidth and exhibiting very low sidelobes is desirable, therefore, because the spectral content of the acoustic signal of such a constant beamwidth transducer is independent of the bearing of the transducer. Also, most directional sound projectors feature substantial sidelobes in their beam patterns. Since these sidelobes are unwanted for most applications, a transducer with negligible sidelobes is desirable.
A number of authors (R. P. Smith, Acustica 23, 21-26 (1970); D. G. Tucker, Nature (London) 180, 496 (1957); J. C. Morris and E. Hands, Acustica 11, 341-347 (1961); and J. C. Morris, Journal of Sound and Vibration 1, 28-40 (1964)) have developed CBT's but these transducers include arrays of elements which are either interconnected by elaborate filters (R. P. Smith), compensating networks (R. P. Smith), or delay lines (D. G. Tucker; J. C. Morris and E. Hands), or are deployed in a complicated three-dimensional pattern (J. C. Morris) thereby making the transducers more suitable as receivers than transmitters. Moreover, all of these transducers exhibit constant beamwidths over a limited bandwidth.
Most directional transducers exhibit a complicated acoustic pressure distribution in the region near the transducer. Such a pressure distribution changes rapidly with the distance from the transducer. Many applications of these transducers require that the observation point be in the rapidly changing region. However, this creates substantial difficulties in correctly interpreting the resulting data. It is desirable, therefore, to have a directional transducer which produces an acoustic pressure distribution that is virtually independent of the distance from the transducer and thereby eliminates any regions having a rapid change in pressure distribution in the near field.
Many directional piezoelectric sound projectors feature a 6 dB rise in transmitting current response (TCR) for each octave increase in frequency below resonance. However, it is desirable to produce for all input frequencies the same level of acoustic pressure amplitude for a given input current and such a constant level requires a flat TCR with respect to frequency. To obtain such a TCR for many transducers, the input current to those transducers must be compensated.